Erosion-corrosion Mechanism And Hydrodynamic Characteristics At An Elbow Of Pipelines

Elbow,which connects pipelines together,is an integral configuration of pipeline systems.However,due to the complexity of geometry of elbow,abrupt diversion in the flowing and moving direction of fliud and sand particles in an elbow would cause severe erosion-corrosion damage,leading to serious wall thinning even perforation at elbow,resulting in deterioration and premature failure of pipelines,and bringing about mass economic loss and severe security risks.Therefore,it is of great significance and value for theory and engineering application to study the mechanism of erosion-corrosion at an elbow,providing the valuable guidance for control and protection of erosion-corrosion at elbow.In present work,a 90-degree elbow was taken as a research object.Array electrodes were mounted into the elbow testing section and erosion-corrosion testing was performed in a circulating loop system to systematacially study the mechanism of erosion-corrosion at the elbow.Weight loss method,electrochemical measurements,surface analysis and computational fluid dynamics(CFD)simulation were also used.The main works are as follows:1.A novel method by combining array electrode technique with CFD simulation was proposed to investigate flow accelerated corrosion(FAC)of an X65 carbon steel elbow under single phase flow.It is demonstrated that corrosion rate at the inner wall of an X65 carbon steel elbow is higher than that at the outer wall of elbow,with the higher flow velocity and shear stress at the inner wall.The maximum corrosion rate appears at innermost side of the elbow,i.e.,the location with the maximum flow velocity and shear stress,while the minimum corrosion rate appears at outermost side of the elbow,the location with minimum flow velocity and shear stress.2.Erosion-corrosion(E-C)behavior of X65 carbon steel elbow under formation water-sand two phase flow was investigated by array electrodes technique and CFD simulation.The four components(pure corrosion rate,pure erosion rate,erosion-enhanced corrosion rate and corrosion-enhanced erosion rate)of E-C rate and the percentages of these four components at different locations of the elbow were firstly quantified.It is demonstrated that the total E-C rate at the outer wall is higher than that at the inner wall of the elbow.The maximum E-C rate appears at the outer wall of elbow outlet.The pure erosion rate and erosion-enhanced corrosion rate account for large part of the total E-C rate.The pure corrosion rate and corrosion-enhanced erosion rate account for small part of the total E-C rate.CFD simulation indicates that the sand concentration is quite high at the bottom but low at the top in upstream straight pipe.At the elbow,sand concentration is much higher at the outer wall than that at the inner wall.The comparison of E-C testing with CFD simulation indicates that a high E-C rate is always associated with a high impact of sand particles.3.The effects of hydrodynamics,concentration of oil-soluble imidazoline inhibitor and flow velocity on inhibition under FAC condition,as well as the inhibition of water-soluble imidazoline inhibitor on erosion-corrosion and its four components at an X65 carbon steel were investigated by array electrode technique.For FAC,it is demonstrated that the inhibition efficiency of oil-soluble imidazoline inhibitor at the inner wall of the elbow is lower than that at the outer wall.The inhibition efficiency at different locations increases with increasing inhibitor concentration when the inhibitor concentration less than 100 mg/L,but decreases in the solution with 200 mg/L inhibitor.The inhibition efficiency decreases with an increasing flow velocity due to drastic turbulence flow and high wall shear stress at a high flow velocity.For E-C,water-soluble imidazoline inhibitor can effectively inhibit the E-C and its four components.The inhibition of corrosion is more pronounced than that of erosion.The IE of total E-C at the outer wall of the elbow is lower than that at the inner wall.The minimum IE of total E-C appears at the outermost side.The IEs of pure erosion,erosion-enhanced corrosion and corrosion-enhanced erosion are higher at the inner wall.The inhibition of pure corrosion and erosion enhanced corrosion is higher than that of pure erosion and corrosion-enhanced erosion.4.The passivation behavior and semiconducting properties of passive films at different locations of a 304 stainless steel elbow under E-C conditions were studied by using array electrodes.It is demonstrated that erosion damage contributes large part of the E-C of stainless steel.The difference in the concentration and impact velocity of sand particles at different locations of the elbow leads to the difference in E-C rate.The passive films formed at the outer wall of elbow outlet are less stable than those formed at inner wall of the elbow and at the outer wall of elbow inlet,which is demonstrated by the more negative corrosion potential and higher passive current density at the outer wall of elbow outlet.Passive films formed on stainless steel elbow under E-C conditions are n-type semiconductor with higher donor density and diffusivity,and a more negative flat band potential at the outer wall of the elbow.Corrosion potential and galvanic current density distributions demonstrate that there is galvanic effect at the different locations of the elbow.The electrodes at the outer wall of elbow inlet and at the inner wall act as cathodes while the electrodes at the outer wall of elbow outlet as anodes,which accelerates the anodic dissolution of the electrodes at the outer wall of elbow outlet.The distributions of the measured(erosion-)corrosion rates and the inhibition effects of inhibitors for FAC and E-C are in good accordance with the hydrodynamics at the X65 carbon steel elbow.The passivation behavior,semiconducting properties of passive films and galvanic effect of stainless steel elbow are associated with the hydrodynamics and sand particles distribution at the 304 stainless steel elbow.